The present invention relates to a finishing process for durable press ("wash and wear") fabrics and, more particularly, to an oxidative afterwash treatment for durable press fabrics containing cellulosic fibers which have been treated with a non-formaldehyde finishing agent.
It is well known that untreated cellulosic fabrics, such as cotton, generally exhibit poor resilience in that they crease or wrinkle easily when crushed. They also have poor dimensional stability and relatively low resistance to shrinkage. In order to overcome those shortcomings, the textile industry has developed various finishing processes to improve the stability and resilience of fabrics containing cellulosic fibers.
A known conventional method for durable press fabrics treats the fabric with a chemical agent that cross-links the cellulose molecules. The chemical reaction is carried out by heating to elevated temperatures to effect the "curing" or cross-linking of the finishing agent and thereby impart crease-resistant properties to the fabric. Many of the commercially-available processes employ, for example, a reactive resin based on formaldehyde such as dimethyloldihydroxyethyleneurea (DMDHEU), a formaldehyde-based resin formed from formaldehyde, glyoxal and urea. Other agents employing nitrogenous cellulose cross-liking agents such as N-methylolamides, are also used in conventional "formaldehyde-based" finishing operations.
In recent years, the concern over the safety of exposure of humans to formaldehyde vapors has resulted in the development of certain non-formaldehyde finishing agents for durable press fabrics such as glyoxal (C.sub.2 H.sub.2 O.sub.2) and non-formaldehyde polymers based on glyoxal such as Sun Chemical's Permafresh ZF, and 1,3-dimethyl-4,5-dimethoxyethyleneurea. Recent studies have shown, for example, that glyoxal is one of the few readily available non-nitrogenous cellulose cross-linking agents that exhibit the high reaction rates required for durable press finishing of cotton. As such, glyoxal offers an attractive alternative to the use of N-methylolamides which liberate formaldehyde vapors during fabric treatment, garment fabrication and apparel use. See Welch, "Glyoxal as a Formaldehyde-Free Durable Press Reagent for Mild Curing Applications", Textile Research Journal, March 1983. However, the use of glyoxal on cellulose containing fabrics in the presence of a catalyst such as aluminum sulfate tends to cause appreciable fabric yellowing and high strength losses. The yellowing can be suppressed, but only to a limited extent, by adding ethylene glycol to the treating formulation to thereby reduce the presence of unreacted aldehyde or hemiacetal groups in the treated fabric. The addition of ethylene glycol also raises the durable press appearance rating of the treated fabric, and increases the resistance of the fabric to laundering abrasion.
Although the use of glyoxal or glyoxal-based polymer finishing agents would reduce the release of formaldehyde vapors during fabric treatment and use, it has been found that the use of such finishes causes moderate to severe yellowing of finished fabrics, particularly upon prolonged storage. That is, fabrics treated with glyoxal will discolor with age and/or exposure to atmospheric contaminants upon storage. A probable explanation for the discoloration is that residual aldehyde and hemiacetal groups interact in the presence of sulfur dioxide, oxygen, ozone, oxides of nitrogen, etc. to produce the undesired yellowing over an extended period of time. Thus, the oxidation of the residual aldehydes to carboxylic acids and cleavage of the hemiacetals may effectively prohibit the formation of color in the fabric.
It has now been found that the discoloration resulting from the use of non-formaldehyde permanent press finishing agents such as glyoxal, polymers of glyoxal and higher aldehydes, particularly the problem of fabric yellowing upon storage over long periods of time, may be eliminated through the use of an oxidative treatment of the treated fabric either simultaneously with or following the durable press finishing operation.
The oxidative treatment according to the present invention restores fabric shade and eliminates yellowing during storage in that it exposes a moist finished fabric to an oxidation solution at an elevated temperature, followed by neutralization, rinsing, and drying operations. The oxidation bath is thought to react with color forming sites in the fabric, thereby eliminating yellowing caused by further reaction at those sites. The oxidative treatment may be performed either during or immediately after curing of the finished fabric in a continuous process, or at a later time as a totally separate process. The equipment necessary to carry out the oxidation can include various combinations of conventional washboxes, steamers and recouperators, so long as the fabric dwell time, oxidant concentration and chemical reaction are adequate to eliminate the undesired yellowing and tendency to discolor upon storage.
A typical oxidation solution according to the invention consists of an oxidant, a stabilizer to retard degradation of the oxidant, a base to control the pH of the oxidative solution, a nonionic surfactant (acting as a wetting agent) and a chelating agent, if desired. The stabilizer and/or base may be optional depending on the choice of the oxidant. Specific but non-limiting examples of oxidants include sodium perborate, sodium percarbonate, sodium peroxide, hydrogen peroxide, sodium perchlorate, peroxyphosphates, persulfates, potassium periodate, and organic per-acids such as peracetic acid. The stabilizer may be any of those known to stabilize bleach baths such as sodium silicate, phosphates, phosphate esters, magnesium salts, phenols, amines, amides, free-radical scavengers or combinations thereof. Likewise, the base, surfactant, and chelating agent may be any of those commonly used in textile bleaching operations.
A typical formulation for an aqueous oxidative afterwash solution according to the present invention is as follows:
sodium percarbonate: 3-4% (by weight of entire bath) PA0 sodium silicate: 0.5-1% PA0 surfactant: 0.1%
The fabric is exposed to the oxidation solution at a temperature between 40.degree. and 200.degree. F. for a period of 5 seconds up to 5 minutes, depending on the strength (concentration) of the oxidation bath. In the preferred embodiment of the invention, the temperature range is between 120.degree. to 160.degree. F. for a period of between 5 and 90 seconds. The fabric is then neutralized with a dilute organic acid (such as dilute acetic acid) rinsed in an aqueous bath, and dried.
The process according to the invention offers a clear advantage over conventional durable press finishing operations in that it provides an efficient and safe method of obtaining a non-formaldehyde durable press fabric which is not discolored after finishing and which will not yellow or become discolored even after extended storage.
Thus, it is an object of the present invention to provide for an efficient and environmentally safe method to eliminate the discoloration resulting from the use of non-formaldehyde durable press finishing agents on cellulosic fibers, particularly the yellowing which occurs upon prolonged storage.
It is a further object of the present invention to provide an afterwash treatment which will eliminate discoloring but not effect the stability, resilience, durability or strength characteristics of the treated fabric.
It is still a further object of the present invention to provide for a continuous oxidative afterwash treatment simultaneously with the durable press finishing which will eliminate discoloring and the tendency to discolor.
Other features, objects and advantages of the invention will appear more fully from the following description of illustrated embodiments taken in conjunction with the appended drawings.